It is the usual practice to provide a transformer in the circuit which interfaces a telephone answering machine with the telephone line. This transformer serves as a direct current isolating means, and also as a means for transforming the single ended audio output of the telephone answering machine to a balanced output for application to the telephone line, and vice versa.
The transformer in the prior art interface circuit also serves to provide the proper direct current and alternating current terminating impedance for the telephone line. For example, most telephone systems require that the value of the direct current terminating impedance must be kept between 90 and 275 ohms, and that the audio signal alternating current terminating impedance be of the order of 600 ohms. These requirements are met in the prior art interface circuit by using a 1:1 turns ratio transformer with, for example, a 600 ohm resistor in its secondary circuit. The direct current primary resistance of the transformer is given a selected value to provide the required 90-275 ohm direct current terminating impedance for the telephone line; and for audio signals, the resistance of the 600 ohm resistor is reflected into the primary circuit of the transformer to provide the required alternating current terminating impedance for the line. However, the transformer used in the prior art interface circuits is bulky, heavy and expensive, and it also adversely affects the quality of the incoming and outgoing audio signals. The circuit of the present invention achieves all the criteria of the prior art circuit described above, but does not require a transformer, so that all the disadvantages of the transformer, as set forth above, are eliminated.